Background Science Information

Background Science Information

Plate Tectonics

The current theory of Plate Tectonics has been developing over the last century beginning with the theory of continental drift first developed by German astronomer, meteorologist, and climatologist Alfred L. Wegener. He was one of several geologists from the early 19th through the 20th centuries that were intrigued by the seeming fit between the Atlantic coasts of South America and Africa.

In 1912, Wegener proposed that millions of years ago the Earth’s continents were one large continent that broke apart and drifted to where they are now located. He created the name Pangaea for this supercontinent. In addition to matching coastlines of the two continents of Africa and South America, Alfred Wegener also used fossil distribution and lithological similarities as evidence.

Fossil distribution

The geographical distribution, layering, and relative frequency of fossils can be compared in different parts of the Earth. The fossil distribution of many plants and animals provides evidence of how they lived at the same time on different continents, such as South America and Africa. The evidence Wegener collected supported the concept that these areas were once joined.

Lithology

The study and description of the macroscopic features of a rock, such as texture and structure, is called lithology. Lithological similarities between rocks in different parts of the world indicate that the same geologic processes were occurring at about the same time—again providing evidence that these areas were once connected.

As with many new scientific ideas, Wegener’s theory was not initially accepted. Some scientists thought that the evidence was not strong enough to support the theory, nor was the underlying cause of the drift of the continents explained; some claimed it was simply impossible.

Two other theories were prevalent in Wegener’s time. One theory stated that the continents and basins were fundamentally unchanged in their position and configuration since their formation. Those who accepted this theory were called “Permanentists.” The opposing belief was that, as the result of a gradual contraction of the solid earth, parts of the ocean floor became dry land, and dry land, in turn, became ocean floor. These scientists were called “Contractionists.”

One enthusiastic supporter of Wegener, however, was South African geologist, Alexander Du Toit. He spent five months in South America collecting fossil evidence in Brazil, Uruguay, and Argentina that he then compared to fossils he collected in Africa. He found that the samples from both South America and Africa contained the same plant and animal fossils embedded in rock, layer-by-layer, in the same complex sequence.

Du Toit eventually proposed that at one time there were two supercontinents: a southern one he called Gondwanaland that consisted of South America, Africa, Antarctica, Australia and India; and a northern supercontinent called Laurasia consisting of North America, Europe, the regions around the Arctic, and the rest of Asia.

Still, it took the development of two new fields of science - paleomagnetism and oceanography - to give the strongest support to both Wegener’s and Du Toit’s theories.

Paleomagnetism

This field of study is based on the principle that, in molten igneous rocks and unlithified sediments, magnetic particles (iron ore) will align themselves with the Earth’s magnetic field. When the igneous rocks containing the magnetic particles cool and the sediments lithify, the magnetic record is stored within the rocks.

British experimental physicist Patrick Blackett developed a sensitive device called the astatic magnetometer in the 1940s. With this instrument, it became possible to detect the orientation of very weak magnetic fields, allowing researchers to conduct paleomagnetic studies of rocks whose magnetism could not previously be measured.

When compared to the current direction of the Earth’s magnetic field, changes in the alignment of the magnetic particles in different layers of rocks showed that the particles were in different positions at some time in the past. Since the rock layers were, and are, part of the continents, the inference, then, is that the continents have been moving.

Oceanography

In 1932, Harry Hess became a geology professor at Princeton University, and later, during WW II, he served as a Navy commander. One of the ships he commanded was equipped for sounding the ocean floor using echo-sounding technology. His wartime duties included searching for enemy submarines, and his equipment enabled him to do his job. During the long voyages to new locations, he and his crew used the same equipment to collect data that became useful to him as a geologist.

In the early 1960s, as the theory of plate tectonics became more accepted throughout the scientific community, Hess thought of studying the data he had collected during WW II while in the Pacific. He examined the data for evidence that might connect loose ends in the plate tectonics theory, and spent a considerable amount of time studying the sounding records gathered as his ship had crisscrossed the Pacific. The results provided him with the first good images of ocean floor characteristics.

Hess gathered additional substantial information about the midoceanic ridges and volcanic valleys of both the Atlantic and Pacific Oceans through further exploration. His meticulous record–keeping and his ability to analyze the data he collected enabled Hess to contribute one of the most influential theories in the geological sciences of the 20th century.

This momentous contribution explained the mechanics of how the continents might have moved as Wegener theorized in 1912. Hess wrote a paper explaining his ideas in 1962, after years of intensive study of the data he collected during WW II. He proposed that molten magma from beneath the earth's crust could ooze up between the plates in the Great Global Rift. As this hot magma cooled in the ocean water, it expanded and pushed the plates on both sides of it -- North and South America to the west and Eurasia and Africa to the east. This way, the Atlantic Ocean widens, but the shapes of the continental coastlines did not change dramatically.

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